1. Field Of The Invention
The present invention relates to high-pressure presses for the hydrostatic extrusion of tubes, especially from heated materials and in particular relates to a new and improved mandrel useful in connection with such presses.
2. Description Of The Prior Art
The high-pressure extrusion presses to which the present invention relates generally comprise a pressure chamber defined by a high-pressure cylinder having a pressure-generating piston inserted in one end thereof and a die opening disposed at its other end. Generally a mandrel is arranged in the pressure chamber and is maintained in a fixed axial position relative to the die opening by appropriate support members. The mandrel includes a tip which projects into the die opening to present an annular gap for shaping a tubular billet into a tube with dimensions determined by the dimensions of the gap when the billet is subjected to a high, all-sided hydrostatic pressure in a surrounding pressure medium. Presses of the type described and their modes of operation are disclosed in greater detail in U.S. Pat. No. 3,751,958, the entirety of the disclosure of which is incorporated herein by reference.
When using presses as described above for extruding heated billets, the mandrels are subjected to very great stresses and the wear of mandrel tips has caused severe problems. In this regard it is pointed out that in the past it has not been practical, either technically or economically, to provide a complete mandrel which is constructed of materials suitable for withstanding heat shocks and having sufficient wear resistance to provide a desirable service life. Thus, two-piece mandrels comprising a carrier of one material and a tip of a different, durable, heat-resistant material have been used.
In such two-piece mandrels the attachment of the tip to the carrier involves a number of problems. Particularly when a mandrel is intended for use in connection with hydrostatic extrusion processes, the working pressures involved are so high that differences in the moduli of elasticity of the different materials must be considered in the design of the mandrel. Moreover, in the construction of a composite mandrel, particular measures must be taken to prevent pressure medium from penetrating between the tip and the carrier at the attachment means during the extrusion operation. Such penetration could cause inadvertent separation of the tip from the carrier. Composite mandrels useful for hydrostatic extrusion operations are described in greater detail in U.S. Pat. Nos. 3,992,913 and 3,994,149, the entirety of the disclosures of which are incorporated herein by reference.
To minimize the overall cost of the composite mandrel, the tip should easily be replaced so that the length of the period of use of the expensive carrier may be maximized. Desirably, a threaded joint might be provided. However, in the past it has been found to be difficult to use a threaded joint for attaching the tip to the carrier and at the same time obtain satisfactory exchangeability because the tips tend to become squeezed tightly in the carrier such that separation is difficult.